1. Field of the Invention
The present invention relates to an antenna coil used in the receiving system of e.g., a keyless entry system of an automobile and a security system, and to a manufacturing method thereof.
2. Related Art
In an antenna coil used in a keyless entry system which is often mounted on an automobile or the like and the receiving system of a security system, a conventional technique in which a plurality of antenna coils wound around rod-like ferrite cores are used so that each of the antenna coils is arranged so as to receive an electric wave in each direction has been known (for instance, see Japanese Patent Application Laid-Open No. 2002-217635). In addition, a technique of forming a small antenna coil using a three-axis core for receiving electric waves in all directions has been disclosed (for instance, see Japanese Patent Application Laid-Open No. 2003-92509). The antenna coil using a three-axis core is shown in FIGS. 8 and 9.
FIG. 8 is a perspective view of a conventional antenna coil, and FIG. 9 shows a three-axis coil 20 configuring FIG. 8. The three-axis coil 20 has a Y-axis coil 25 which is wound in a Y-axis groove 22 which passes through the center of the upper surface and the lower surface of a flat columnar three-axis core 21 made of a ferrite material and is provided so as to divide an outer circumference from the upper surface to the lower surface into two and in which a Y direction is determined to be a winding axis direction, an X-axis coil 26 which is wound in an X-axis groove 23 which is orthogonal to the Y-axis groove 22, passes through the center of the upper surface and the lower surface of the three-axis core 21, and is provided so as to divide the outer circumference from the upper surface to the lower surface into two and in which an X direction is determined to be a winding axis direction, and a Z-axis coil 27 which is wound in a Z-axis groove 24 provided on the outer circumference of the three-axis core 21 and in which when the thickness direction of the three-axis core 21 is determined to be a Z direction, the Z direction is determined to be a winding axis direction.
As shown in FIG. 8, the three-axis coil 20 is housed in a resin case 28 in which four external terminals 29 are provided on opposite side surfaces. In addition, the end of each of the windings of the X-axis coil, the Y-axis coil, and the Z-axis coil is electrically connected to the predetermined external terminal 29.
In the above three-axis coil 20, flange portions at both ends of the Z-axis groove 24, for receiving an electric wave in the Z-axis direction, are divided into four in order to form the X-axis groove 23 and the Y-axis groove 22. In other words, the width of the X-axis groove 23 and the width of the Y-axis groove 22 reduce the area of the flange portions at both ends of the Z-axis coil 27. For this reason, the characteristic of the Z-axis coil 27 is lower than that of the X-axis coil 23 and the Y-axis coil 25. To improve this problem, the number of turns in the Z-axis coil 27 may be increased or the thickness of the three-axis core 21 may be increased, but the shape of the antenna coil is increased, and thinning is inhibited.
In addition, because the X-axis groove 23 and the Y-axis groove 22 are provided so that the winding axis directions of the X-axis coil 26 and the Y-axis coil 25 are orthogonal to each other, the areas of the flanges of the X-axis coil and the Y-axis coil are reduced, thereby deteriorating the characteristic. In other words, the areas of the flanges at both ends of the X-axis groove 26 divided into two by the Y-axis groove 22 are reduced due to the width of the Y-axis groove 22. In other words, the characteristic of the X-axis coil 26 is deteriorated by the reduced area of the flange of the X-axis coil. Likewise, the characteristic of the Y-axis coil 25 is also deteriorated.
Further, when the respective windings are close to or contact each other in the uppermost portion of the winding (the winding end portion) of the X-axis coil 23 and the lowermost portion of the winding (the winding start portion) of the Y-axis coil 22, capacitive coupling occurs. The capacitance of the capacitive coupling is changed according to the connecting method of the X-axis coil 26, the Y-axis coil 25, and the Z-axis coil 27, thereby deteriorating the characteristic of the antenna coil.